1. Field of the Invention:
This invention relates in general to seismic exploration systems and in particular to seismic exploration systems which incorporate techniques and methods for cancelling non-uniformly distributed noise signals.
2. Description of the Prior Art:
In known seismic exploration systems, seismic waves are typically induced by a plurality of pulses of sonic vibration which are generally applied over a predetermined period and which generally have a predetermined frequency sequence. The applied vibration penetrates the earth so that reflections from deep layers can be detected by sensing devices or geophones at the earth's surface. These reflections can then be related to the geologic layering within the earth. Sonic vibrations applied to the earth's surface also generate a so-called "surface wave" which radiates in a shallow layer in all directions from the point of application. Sensing devices or geophones positioned on the earth's surface which are intended to receive waves from deep reflecting layers within the earth also receive this surface wave. The reflected waves from deep within the earth's surface are of great interest and the presence of surface waves, as well as other noises, provide interference which make it difficult to accurately ascertain information concerning the geologic layering within the earth.
This problem is exacerbated by the fact that surface waves demonstrate a much higher amplitude than reflected waves and the arrival of a surface wave at a particular geophone may occur simultaneously with the arrival of the desired reflected waves.
Several methods are known in the prior art for limiting the aforementioned interference from surface waves. Typically, geophones are not utilized individually but rather are connected together in sub-arrays which are added or averaged by connecting the geophones in series and parallel combinations or completely in parallel or completely in series. These sub-arrays are then deployed along the surface of the earth over a distance of one or more wavelengths of the surface waves. For example, if the applied vibrations are of a frequency modulated sinusoidal wave of finite duration sweeping, for example, in frequency from ten hertz to seventy-five hertz, the longest wavelength would occur at ten hertz. If the speed of propagation of the surface wave is between one thousand and two thousand feet per second, then the ten hertz wavelength would be between one hundred and two hundred feet. If the sub-array is spread so that its elements extend over two hundred feet or more on a radial line from the vibration source, substantial cancellation of the surface waves may take place as the sub-array geophone signals are added to one another.
This technique causes some cancellation of surface waves without affecting the seismic waves arriving from deep reflecting layers. These reflected waves arrive at the surface at angles close to vertical and since their apparent horizontal propagation speed is much higher than the propagation speed of surface waves, the difference in arrival time among the sub-array geophones is negligible and cancellation of deep arriving waves does not take place. Thus, the utilization of sub-arrays of geophones enhances the ratio of reflected wave signals to direct surface wave signals over that which could be obtained utilizing single geophones.
Another method of cancelling undesired interference utilizes matched filtering to cut down on random noise received from within the earth and also to cut down on vibrations received via the surface wave from the source of vibrations. For example, a low frequency cutoff of sweep bandwidth will act to attenuate the surface wave. The known transmitted waveform taken from the source of vibration is cross-correlated in a cross-correlator with the signal output of the geophone sub-array. The time lag between the known transmitted waveform and the received geophone sub-array signal is set in the cross-correlator to correspond to the two-way travel time from the surface down to the target reflector zone and then back to the surface. In the absence of surface waves, the cross-correlation corresponds to the amplitude of the reflected wave from the target depth. Random noise from the earth tends to be eliminated by this technique because it does not correlate with the transmitted waveform. Also, reflected energy from depths other than the target depth do not correlate with the transmitted waveform when it is time lagged for a particular target depth. However, some correlation which is non-zero will result from the surface waves which will severely interfere with the correlation of the reflected wave with the transmitted wave. This would generally not be a problem except for the fact that the amplitude of the surface wave is so much greater than the amplitude of the reflected wave and the uncorrelated recording of data includes the interference of noise throughout the record.
Recently, U.S. Pat. No. 4,556,962, issued to Widrow, teaches a method of cancelling surface waves which utilizes adaptive filtering techniques to attempt to cancel the interference caused by surface waves. This method utilizes one receiving seismic detector which is placed on the surface of the earth near the source of sonic vibrations to generate a reference signal which is representative of the applied seismic wave. This reference signal is then adaptively filtered and combined with the outputs of various seismic detectors to subtract the surface wave signal from the detector outputs to provide an output signal which is representative of the seismic waves reflected from deeper formations.
While this latest technique represents an improvement in the art, it does not address the problems associated with machinery noise, wind noise or airwave and it does not address the fact that surface wave interference is not uniformly distributed throughout an area of investigation. That is, the interference caused by the surface wave at one location may be substantially different from the interference which occurs at a second location due to anomalies within the surface of the earth or other variable parameters. Similarly, airwave and wind noise may vary substantially from one portion of the area of investigation to a second portion due to physical characteristics of the area of investigation or the direction of the wind. And, of course, those skilled in the art will recognize that machinery noise caused by compressors, rock crushers, or pumps may dramatically affect the accuracy of seismic measurements at locations within the area of investigation which are close to the source of such mechanical noises.
Therefore, it should be obvious that a need exists for a seismic exploration method and apparatus which can be utilized to effectively cancel non-uniformly distributed noise throughout a wide area of investigation.